1. Field of the Invention
The present invention relates to bearings of shaft and more particularly to a method and apparatus for automatically dynamically adjusting prestressed bearings of shaft.
2. Description of the Prior Art
As understood that how to develop and produce products having industrial applicability is an important thing to manufacturers. This is particularly true in the competition of machine tool manufacturing. In the case of high speed shaft of machine tool, now the manufacturing thereof is at a bottleneck due to higher precision requirement and wide applications. Such problem is caused by heat accumulation on shaft and bearings thereof during rotation, i.e., heat dissipation is not enough.
FIG. 1 is a block diagram showing heat generated in major components during rotation of a conventional shaft and bearings thereof. As shown, for maintaining rigidity and precision of shaft, a prestress is exerted on each bearing of shaft prior to rotation. This can increase friction between bearings and shaft. Accordingly, excessive heat is generated. Such excessive heat is partly transferred from motor. As such, components such as bearings or sleeve assemblies are expanded due to the heat. And in turn, prestress on bearings is increased. As a result, friction between bearings and shaft is further increased. Such cycle is repeated during the rotation of shaft. Eventually, bearings are malfunctioned.
A couple of techniques have been proposed to solve above problem:
1. Prestressed spring mounting: A compressed spring is coupled to bearing of shaft. Hence, a potential deformation of bearing may be absorbed by the expansion of spring during the high temperature operating environment. However, the previous design suffered from several disadvantages. For example, rigidity of shaft may be adversely affected. Also, it is difficult to control the precision of shaft. Moreover, it is not easy to choose an optimum spring having a desired elasticity coefficient, resulting in a poor reliability and non-uniform prestress distribution of bearing. Such spring-based improvement is limited in applications.
2. Hydraulic prestressed means: Hydraulic prestressed switching devices are most popular. However, this design still suffered from several disadvantages. For example, an additional hydraulic oil supply and associated equipment are required. This can increase cost. Further, such device is frequently malfunctioned. Furthermore, useful space is occupied by the device. Moreover, prestress of oil supply is subject to change during oil pumping cycle. To the worse, bearing may be damaged if there is a change of temperature of oil supply and/or oil leaking.
Thus, it is desirable to provide a method and apparatus for automatically dynamically adjusting prestressed bearings of shaft in order to overcome the above drawbacks of prior art.
It is therefore an object of the present invention to provide a method for automatically dynamically adjusting prestressed bearings of a shaft comprising the steps of a) forming a sleeve assembly comprising an outer sleeve and a first inner sleeve fitted together; b) selecting a thermal expansion coefficient of the outer sleeve larger than that of the first inner sleeve; c) machining surfaces of the outer and the first inner sleeves for making an inner diameter of the outer sleeve smaller than an outer diameter of the first inner sleeve; d) heating the outer sleeve at a predetermined temperature for expanding the inner diameter thereof to be larger than the outer diameter of the first inner sleeve prior to putting the outer sleeve onto the first inner sleeve; e) decreasing temperature of the outer and the first inner sleeves for securing them together; and f) mounting the sleeve assembly onto the shaft prior to sandwiching the sleeve assembly between the bearings, thereby exerting a predetermined prestress on each of the bearings. By utilizing this method, prestress reduction of bearings of shaft without degrading a rigidity of bearings is carried out. Further, no additional complex equipment is needed, resulting in a reduction in the manufacturing and maintenance costs and space saving.
It is another object of the present invention to provide an apparatus for automatically dynamically adjusting prestressed bearings of a shaft comprising a sleeve assembly sandwiched between the bearings and including an outer sleeve and a first inner sleeve fitted together wherein a thermal expansion coefficient of the outer sleeve is larger than that of the first inner sleeve, an inner diameter of the outer sleeve is smaller than an outer diameter of the first inner sleeve, the outer and the first inner sleeves are secured together in a room temperature, and the first inner sleeve is compressed by the outer sleeve to extend axially.